1. Field of the Invention
The present invention relates to an axial multi-piston compressor comprising a drive shaft, a cylinder block having cylinder bores formed therein and surrounding the drive shaft, and a plurality of pistons slidably received in the cylinder bores, respectively, wherein the pistons are successively reciprocated in the cylinder bores by a rotation of the drive shaft so that a suction stroke and a discharge stroke are alternately executed in each of the cylinder bores.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 59(1984)-145378 discloses a swash plate type compressor as representative of an axial multi-piston compressor, which may be incorporated in an air-conditioning system used in a vehicle such as an automobile. This swash plate type compressor comprises: front and rear cylinder blocks axially combined to form a swash plate chamber therebetween, the combined cylinder blocks having a same number of cylinder bores radially formed therein and arranged with respect to the central axis thereof, the cylinder bores of the front cylinder block being aligned and registered with the cylinder bores of the rear cylinder block, respectively, with the swash plate chamber intervening therebetween; double-headed pistons slidably received in the pairs of aligned cylinder bores, respectively; front and rear housings fixed to front and rear end faces of the combined cylinder blocks through the intermediary of front and rear valve plate assemblies, respectively, the front and rear housings each forming a suction chamber and a discharge chamber together with the corresponding one of the front and rear valve plate assemblies; a rotatable drive shaft arranged so as to be axially extended through the front housing and the combined cylinder blocks; and a swash plate securely mounted on the drive shaft within the swash plate chamber and engaging with the double-headed pistons to cause these pistons to be reciprocated in the pairs of aligned cylinder bores, respectively, by the rotation of the swash plate.
The front and rear valve plate assemblies in particular have substantially the same construction, in that each comprises: a disc-like member having sets of a suction port and a discharge port each set being able to communicate with the corresponding one of the cylinder bores of the front or rear cylinder block; an inner valve sheet attached to the inner side surface of the disc-like member and having suction reed valve elements formed integrally therein, each of which is arranged so as to open and close the corresponding suction port of the disc-like member; and an outer valve sheet attached to the outer side surface of the disc-like member and having discharge reed valve elements formed integrally therein, each of which is arranged so as to open and close the corresponding discharge port of the disc-like member. Each of the front and rear valve plate assemblies is also provided with suction openings aligned with passages formed in the front or rear cylinder block, respectively, whereby the suction chambers formed by the front and rear housings are in communication with the swash plate chamber into which a fluid or refrigerant is introduced from an evaporator of an air-conditioning system, through a suitable inlet port formed in the combined cylinder blocks.
In the swash plate type compressor as mentioned above, the drive shaft is driven by the engine of a vehicle, such as an automobile, so that the swash plate is rotated within the swash plate chamber, and the rotational movement of the swash plate causes the double-headed pistons to be reciprocated in the pairs of aligned cylinder bores. When each piston is reciprocated in the aligned cylinder bores, a suction stroke is executed in one of the aligned cylinder bores and a compression stroke is executed in the other cylinder bore. During the suction stroke, the suction reed valve element is opened and the discharge reed valve element is closed, whereby the refrigerant is delivered from the suction chamber to the cylinder bore through the suction port. During the compression stroke, the suction reed valve element concerned is closed and the discharge reed valve element concerned is opened, whereby the delivered refrigerant is compressed and discharged from the cylinder bore into the discharge chamber, through the discharge reed valve element.
In this type compressor, the refrigerant includes a lubricating oil mist, and the movable parts of the compressor are lubricated with the oil mist during the operation. Also, the oil mist appears on the suction and discharge reed valve elements, and serves as a liquid-phase seal when each of the reed valve elements is closed.
When the compression stroke is finished in each of the cylinder bores, the corresponding discharge reed valve element is closed. At this point of time, a small part of the compressed refrigerant is inevitably left in a fine space defined between the piston head and the valve plate assembly and in the discharge port formed in the valve plate assembly, and the corresponding suction reed valve element is adhered to the valve seat thereof with the liquid-phase oil. Accordingly, just after the suction stroke is initiated, i.e., just after the corresponding head of the double-headed piston is moved from top dead center toward bottom dead center, the suction reed valve element cannot be immediately opened, i.e., the refrigerant cannot be immediately introduced from the suction chamber into the cylinder bore through the suction reed valve element, because the residual part of the compressed refrigerant has a higher pressure than that of suction chamber, and because and the adhesion force and resilient force of the suction reed valve must be overcome before the refrigerant can be introduced from the suction chamber to the cylinder bore through the suction port. Namely, at the beginning of the suction stroke, the residual part of the compressed refrigerant is merely expanded in the cylinder bore, and thus the introduction of the refrigerant from the suction chamber into the cylinder bore cannot take place until a differential between the pressures in the cylinder bore and the suction chamber exceeds a certain level.
Therefore, in the compressor as mentioned above, a practical suction volume of the refrigerant, which can be obtained during the suction stroke, is lower than a theoretical suction volume of the refrigerant due to the residual part of the compressed refrigerant, and thus it is impossible to sufficiently realize a theoretical performance from the compressor.
Japanese Unexamined Patent Publication (Kokai) No. 5(1993)-71467, corresponding to U.S. Pat. No. 5,232,349 issued on Aug. 3, 1993, discloses an axial multi-piston compressor constituted such that a theoretical suction volume of the refrigerant can be substantially obtained during the suction stroke. In this compressor, the suction reed valves are substituted for a single suction rotary valve slidably disposed in a central circular space formed in the cylinder block and joined to the drive shaft for rotation thereof. Namely, the valve plate assembly is provided with only the discharge reed valve elements and the discharge ports, and the suction reed valve elements and the suction ports are eliminated therefrom. The suction rotary valve is provided with an arcuate groove formed in a peripheral surface thereof, and the arcuate groove is in communication with the suction chamber. The suction rotary valve is further provided with a through passage extending diametrically therethrough. On the other hand, the cylinder block is provided with radial passages formed therein, and each of these radial passages is in communication with the corresponding cylinder bore at an end face thereof on which the discharge port is disposed. The inner ends of the radial passages are opened at an inner wall face of the central circular space of the cylinder block in which the suction rotary valve is slidably received.
In the compressor as disclosed in JUPP (Kokai) No. 5(1993)-71467 (U.S. Pat. No. 5,232,349), when the suction stroke is executed in each of the cylinder bores, the cylinder bore concerned is communicated with the suction chamber through the radial passage thereof and the arcuate groove of the suction rotary valve, so that the refrigerant is introduced thereinto. During the suction stroke, the communication is maintained between the cylinder bore and the suction chamber due to a given arcuate length of the arcuate groove. When the suction stroke is finished, i.e., when the piston reaches bottom dead center, the communication between the cylinder bore and the suction chamber is cut off. Then, the compression stroke is initiated, so that the piston stroke is moved from bottom dead center toward top dead center. When the compression stroke is finished, i.e., when the piston reaches top dead center, a part of the compressed refrigerant is inevitably left in a small volume of the cylinder bore defined by the piston head and the valve plate assembly, similar to the compressor as disclosed in JUPP (Kokai) No. 59(1984)-145378. However, just after the compression stroke is finished, i.e., just after the piston is moved from top dead center toward bottom dead center, the cylinder bore concerned is communicated with the diametrically opposed cylinder bore, in which the suction stroke is just finished, through the diametrical through passage formed in the rotary valve, and thus the residual part of the compressed refrigerant escapes from the cylinder bore concerned to the diametrically opposed cylinder bore not governed by the compression stroke. Accordingly, as soon as the cylinder bore concerned is made to communicate with the suction chamber through the radial passage thereof and the arcuate groove of the rotary valve, the refrigerant is introduced from the suction chamber the cylinder bore concerned, due to the escape of the residual part of the compressed refrigerant. As a result, a practical suction volume of the refrigerant, which can be obtained during the suction stroke, is substantially equal to a theoretical suction volume of the refrigerant, and thus it is possible to substantially realize a theoretical performance from the compressor.
Nevertheless, in the compressor as disclosed in U.S. Pat. No. 5,232,349, a drive force for moving the piston from top dead center toward bottom dead center becomes larger in comparison with the compressor as disclosed in JUPP No. (Kokai) No. 59(1984)-145378, because the residual part of the compressed refrigerant escapes just after the compression stroke is finished. Namely, existence of the residual part of the compressed refrigerant results in a decrease in the suction volume of the refrigerant, but it contributes to reduction of the drive force for moving the piston from top dead center toward bottom dead center. Accordingly, although a compression performance of the compressor disclosed in U.S. Pat. No. 5,232,349 can be substantially improved, it exhibits a drive efficiency inferior to that of the compressor as disclosed in JUPP No. (Kokai) No. 59(1984)-145378.
Also, the compressor shown in U.S. Pat. No. 5,232,349 involves a problem to be solved. In particular, when the residual part of the compressed refrigerant escapes from the cylinder bore concerned to the diametrically opposed cylinder bore not governed by the compression stroke, a pressure of the escaped part of refrigerant is substantially lowered to a low pressure level in the suction chamber. Thus, the pressure of the escape part of refrigerant must be again raised during the compression stroke. Of course, this results in a great loss in an efficiency of compression in the compressor.